FeO2 and FeOOH under deep lower-mantle conditions and Earth's oxygen-hydrogen cycles
FeO2 and FeOOH under deep lower-mantle conditions and Earth's oxygen-hydrogen cycles
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DOI：
10.1038/nature18018
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发表时间：
2016-06-09
期刊：
影响因子：
64.8
通讯作者：
Mao, Ho-Kwang
中科院分区：
文献类型：
--
作者：
Hu, Qingyang;Kim, Duck Young;Mao, Ho-Kwang
地球内部氧和氢的分布、积累和循环决定了水圈、大气圈和生物圈的地球化学演化（1）。富氧的大气层和富铁的核心代表了氧-铁（O-Fe）系统的两个端元，与行星的整个压力-温度-成分范围重叠。内部深处的极端压力和温度条件改变了铁氧化物的氧化态（1）、自旋态（2）和相稳定性（3，4），产生了新的化学计量，如Fe 4 O 5（参考文献5）和Fe 5 O 6（参考文献6）。O和Fe之间的这种相互作用决定了地球的形成、地核和地幔的分离以及大气的演化。铁在其多种氧化态下控制氧逸度和氧收支，氢在铁和O的反应中起关键作用（导致铁在潮湿空气中生锈）。在这里，我们使用第一性原理计算和实验来确定一种高度稳定的黄铁矿结构氧化铁（FeO 2），其压力为76吉帕斯卡和1,800开尔文，含有过量的氧气。我们发现，矿物针铁矿，FeOOH，普遍存在的“锈”，并集中在沼泽铁矿石，在深下地幔条件下分解，形成FeO 2和释放H-2。该反应可能导致下地幔深部重FeO 2斑块的聚集、氢的向上迁移和氧氢循环的分离。这一过程为下地幔深部地震和地球化学异常的起源提供了另一种解释，也为20多亿年前的大氧化事件提供了零星的O-2来源，该事件创造了目前的富氧大气。
The distribution, accumulation and circulation of oxygen and hydrogen in Earth's interior dictate the geochemical evolution of the hydrosphere, atmosphere and biosphere(1). The oxygen-rich atmosphere and iron-rich core represent two end-members of the oxygen-iron (O-Fe) system, overlapping with the entire pressure-temperature-composition range of the planet. The extreme pressure and temperature conditions of the deep interior alter the oxidation states(1), spin states(2) and phase stabilities(3,4) of iron oxides, creating new stoichiometries, such as Fe4O5 (ref. 5) and Fe5O6 (ref. 6). Such interactions between O and Fe dictate Earth's formation, the separation of the core and mantle, and the evolution of the atmosphere. Iron, in its multiple oxidation states, controls the oxygen fugacity and oxygen budget, with hydrogen having a key role in the reaction of Fe and O (causing iron to rust in humid air). Here we use first-principles calculations and experiments to identify a highly stable, pyrite-structured iron oxide (FeO2) at 76 gigapascals and 1,800 kelvin that holds an excessive amount of oxygen. We show that the mineral goethite, FeOOH, which exists ubiquitously as 'rust' and is concentrated in bog iron ore, decomposes under the deep lower-mantle conditions to form FeO2 and release H-2. The reaction could cause accumulation of the heavy FeO2-bearing patches in the deep lower mantle, upward migration of hydrogen, and separation of the oxygen and hydrogen cycles. This process provides an alternative interpretation for the origin of seismic and geochemical anomalies in the deep lower mantle, as well as a sporadic O-2 source for the Great Oxidation Event over two billion years ago that created the present oxygen-rich atmosphere.